Electrically conducting metallic ground planes have been successfully used for many years in the design of a wide variety of antenna systems. However, there are several major drawbacks associated with using conventional metallic ground planes for antenna applications. These include the fact that 1) horizontally polarized antennas, such as dipoles, must be placed at least a quarter-wavelength above the ground plane in order to achieve optimal performance, and 2) ground planes of this type are known to support surface waves, which are undesirable in many antenna applications.
Recently the concept of an artificial magnetic conductor (AMC) ground plane was introduced as a means of mitigating many of the problems associated with the use of conventional electrically conducting ground planes.
The term artificial magnetic conductor (AMC) typically refers to a structure comprising a dielectric layer having a conducting sheet on one surface and a frequency selective surface (FSS) on the other surface. The FSS is typically an array of conducting patterns supported by a non-conducting surface (the surface of the dielectric layer).
An individual conducting pattern, repeated over the surface of the FSS, may be referred to as a unit cell of the FSS. Conventionally, the unit cell is repeated without variation over the FSS. Typically, the unit cell is a square conducting patch repeated in a grid pattern, for example as described in U.S. Pat. No. 6,525,695 to McKinzie et al. However, more complex shapes are possible.
At a resonance frequency, the AMC behaves as a perfect magnetic conductor, and reflected electromagnetic waves are in phase with the incident electromagnetic waves. This effect is useful in increasing the radiated output energy of an antenna, as radiation emitted backwards from the antenna can be reflected in phase from an AMC backplane, and hence can contribute to the forward emitted radiation, as any interference will be constructive. Hence, the term AMC is given to a multi-component structure providing the properties of a magnetic conductor at one or more frequencies.
Conventional AMC technology is described by D. Sievenpiper, et al., IEEE Trans. Microwave Theory Tech., vol. MTT-47, pp. 2059-2074, November 1999 and F. Yang, et al., pp. 1509-1514, August 1999. Thin AMC ground planes with thicknesses on the order of 1/100 or less of the electromagnetic wavelength can be effectively used to design low-profile horizontally polarized dipole antennas. The use of an AMC in this case allows the antenna height to be considerably reduced to the point where it is nearly on top of the AMC surface. In addition, AMC ground planes also possess the added advantage of being able to suppress undesirable surface waves.
While the conventional AMC ground planes can enhance the performance of many commonly used antennas, they are typically narrow-band and lack the flexibility required for use in low-profile frequency-agile antenna systems.
U.S. Pat. No. 6,483,480 to Sievenpiper et al. describes a tunable impedance surface having a ground plane and two arrays of elements, the one array moveable relative to the other. Int. Pat. Pub. No. WO94/00892 and GB Pat. No. 2,253,519, both to Vardaxoglou, describe a reconfigurable frequency selective surface in which a first array of elements is displaced relative to a second array. U.S. Pat. No. 6,690,327 to McKinzie et al. describes a mechanically reconfigurable AMC. However, mechanical reconfiguration of an array of elements can be difficult to implement.
U.S. Pat. No. 6,469,677 to Schaffner et al. describes the use of micro-electromechanical system (MEMS) switches within a reconfigurable antenna. U.S. Pat. Nos. 6,417,807 to Hsu et al. and U.S. Pat. No. 6,307,519 to Livingston et al. also describe MEMS switches within an antenna. U.S. Pat. No. 6,448,936 to Kopf et al. describes a reconfigurable resonant cavity with frequency selective surfaces and shorting posts. However, these patents are not directed towards a reconfigurable AMC.
U.S. Pat. No. to 6,525,695 and U.S. Pat. App. Pub. No. 2002/0167456, both to McKinzie, describe a reconfigurable AMC having voltage controlled capacitors with a coplanar resistive biasing network. U.S. Pat. No. 6,512,494 to Diaz et al. describes multi-resonant high-impedance electromagnetic surfaces, for example for use in an AMC. Int. Pat. Pub. No. WO02/089256 to McKinzie et al., U.S. Pat. App. Pub. No. 2003/0112186 to Sanchez et al., and U.S. Pat. App. Pub. No. 2002/0167457 to McKinzie et al. describe the control of the sheet capacitance of a reconfigurable AMC. U.S. Pat. No. 6,028,692 to Rhoads et al. describes a tunable surface filter having a controllable element having an end-stub.
Approaches described in the prior art may allow the tuning of a resonance frequency of an AMC, but may not allow the change of other parameters such as resonance width, or allow reconfiguration of multiple band AMCs. Typically, adjustments are made over the whole surface of the AMC, not allowing for local adjustments. Also, reconfigurable pixel configurations are not disclosed.
Patents and published U.S. patent applications referenced in this application are incorporated herein by reference. Co-pending U.S. patent applications to one or more of the present inventors are also incorporated herein by reference, including: U.S. application Ser. No. 10/755,539, filed Jan. 12, 2004, to Werner (concerning metaferrite properties of an AMC); U.S. application Ser. No. 10/625,158, filed Jul. 23, 2003 (concerning fractile antenna arrays); and U.S. application Ser. No. 10,712,666, filed Nov. 13, 2003, to Jackson (concerning a reconfigurable pixelized antenna system).
FIGS. 8A and 8B show an electromagnetic reflector and electromagnetic absorber, respectively. The electromagnetic reflector 180 tends to reflect electromagnetic radiation. The incident radiation is indicated as wavy arrowed line I, and the reflected radiation is indicated by wavy arrowed line R. The electomagnetic absorber 182 tends to absorb electromagnetic radiation, there being no reflected radiation R shown. FIG. 8C shows an antenna 184, having radiative elements 186, and antenna backplane 188. The electromagnetic reflector, electromagnetic absorber, and antenna ground plane are useful components known in the art, and improved devices would allow improved properties.